In the manufacture of devices such as integrated circuits, a pattern is produced through lithograhic processes in a radiation-sensitive material coated on a device substrate. (Substrate, for purposes of this disclosure, is a body, e.g., a semiconductor body, that is being processed into a device or plurality of devices, e.g., semiconductor devices. This body could include not only semiconductor materials or optoelectronic materials, but also regions of organic materials, metals, and/or dielectrics.) These processes generally involve a series of exposures over the radiation-sensitive material, e.g., a resist, in a desired repeated pattern by light projected through a mask or reticle and focused onto the photosensitive material with a suitable lens system. However, the substrate before each exposure must be positioned relative to the projected pattern (generally by use of a translation stage, controlled by a position measuring device such as an interferometer) so an appropriate spatial alignment is obtained between the projected image and device patterns on the wafer.
One method of alignment is a global technique. In this procedure to calibrate the system a sample wafer having appropriately positioned alignment mark(s) underlying a radiation-sensitive layer is, for calibration, moved into the viewing field of a microscope, 31 FIG. 1, external to the exposure lens system. The substrate alignment marks are aligned with the reference fiducial mark(s) of the microscope. The substrate is then moved to the exposure position under interferometric control. The distance of movement is calculated from the known position of the microscope fiducial mark and from the desired position of exposure.
The substrate is then exposed utilizing the reticle to be employed in the device fabrication. The reticle pattern includes an alignment mark (generally a vernier series of marks) that is projected onto the radiation-sensitive material. The radiation-sensitive material is developed and the spatial separation of the projected alignment mark relative to the wafer alignment mark, i.e., the spatial error, is determined by optical inspection. After calibration, for device production, the device substrate being processed is initially aligned using the microscope fiducial marks. The substrate is then positioned for exposure by, for example, interferometry employing a suitable correction for all positioning as previously determined by the global calibration. Subsequent, positioning is done by dead reckoning from one exposure position to the next on the substrate.
The relative large dead reckoning translations associated with the global alignment technique introduces a concomitant error. In a second approach denominated a site-by-site technique, a separate alignment measurement is performed for each exposure on a wafer during production, and thus, less error is typically introduced. In this technique there is an alignment mark(s) on the wafer for each fabrication exposure to be performed. Before each fabrication exposure there is an alignment at the corresponding alignment mark and then movement from the alignment position a known, relatively short, distance to the fabrication exposure position.
A more desirable approach for a site-by-site alignment procedure would use the exposing wavelength to project a fiducial mark on the mask or reticle through the imaging lens with alignment accomplished between this projection and a corresponding mark on the substrate without the necessity of moving the substrate between alignment and fabrication exposure. In this manner, it would be possible to eliminate errors associated with (1) wafer movement and with (2) assumptions based on both the indirect measurement of distance and on the equivalence of different lens systems to different wavelengths. In practice it is difficult to employ this desired approach. It is usually undesirable to employ the exposing wavelength for alignment because the radiation-sensitive material on the substrate generally is irreversibly modified in the region of the alignment marks (fiducial marks). This premature exposure is undesirable because the alignment mark necessary for subsequent alignment in the next device layer is often obliterated. Additionally, deep ultraviolet light is often strongly absorbed by the photosensitive material, and thus, does not illuminate the mark on the wafer.
Since it is generally not desirable to use light of the exposing wavelength for alignment, the use of the exposure lens system is also often precluded and alignment is done at a place removed from the exposure position even in systems that align through the exposure lens. (This limitation is especially severe for deep ultraviolet exposure systems--systems operating with light in the wavelength range 350 to 180 nm.) The exposure lens system is designed for the wavelength of the exposing light, and thus, causes optical distortions when used with a different wavelength. The greater the differrence in wavelength between the aligning and exposing light, the greater the distortion. Generally, longer wavelength light (light in the wavelength range 5000 to 11,000 Angstroms) is used for alignment to avoid exposure of the resist material. Such light, e.g., red light from the HeNe laser, is so far removed from the deep ultraviolet region of the spectrum that gross distortions would occur if the exposing lens system is utilized for alignment. Additionally, the anti-reflection coatings on the exposure lenses often produce large reflections at wavelengths other than the exposure wavelength. Some site-by-site systems compensate for these effects by placing special but small optical elements in the exposure lens. These elements are usually located at the edge of the exposure field where they do not interfere with normal operation. However, for stability, these elements are fixed in position, and to be aligned the alignment mark on a wafer must be moved under these elements and away from the exposure position.
For these reasons, site-by-site alignment is generally done with the device substrate positioned on the edge of the lens system, either outside the lens barrel as shown in phantom at 60 in FIG. 1 or at the very edge of the exposure field, to allow non-distorted introduction of the aligning light. Reflection is observed with a monitor, e.g., a television camera or the microscope. The optical axis or other known fiducial mark position of the microscope is calibrated relative to a point of reference. The reticle fiducial mark is calibrated relative to the same point of reference. The stage is then moved to alignment using interferometric techniques a calculated distance based on the two calibration measurements and optical alignment measurement. Clearly, numerous errors are possible in calibration, measurement, and movement. Thus, although alignment techniques have been satisfactory for presently used wavelengths (the blue and near ultraviolet wavelengths) and/or for design rules of 0.9 .mu.m and larger, improvement is certainly possible. Additionally, for deep ultraviolet lithographic systems serious difficulties and unresolved impediments are presented.